Core Contracts
Regnum Aurum Acquisition Corp
HardhatReal World AssetsNFT
77,280 USDC
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Submission Details
Severity: medium
Valid

Dust amounts can't be swipped out from the RToken

dimah7

Vulnerability Details

The Lending pool has an option to clear accumulated dust amounts from the RToken by calling:

function transferAccruedDust(address recipient, uint256 amount) external onlyReservePool {
if (recipient == address(0)) revert InvalidAddress();
​
uint256 poolDustBalance = calculateDustAmount();
if(poolDustBalance == 0) revert NoDust();
​
// Cap the transfer amount to the actual dust balance
uint256 transferAmount = (amount < poolDustBalance) ? amount : poolDustBalance;
​
// Transfer the amount to the recipient
IERC20(_assetAddress).safeTransfer(recipient, transferAmount);
​
emit DustTransferred(recipient, transferAmount);
}

As we can see it first calculates the dust amount by calling calculateDustAmount:

function calculateDustAmount() public view returns (uint256) {
// Calculate the actual balance of the underlying asset held by this contract
uint256 contractBalance = IERC20(_assetAddress).balanceOf(address(this)).rayDiv(ILendingPool(_reservePool).getNormalizedIncome());
​
// Calculate the total real obligations to the token holders
uint256 currentTotalSupply = totalSupply();
​
// Calculate the total real balance equivalent to the total supply
uint256 totalRealBalance = currentTotalSupply.rayMul(ILendingPool(_reservePool).getNormalizedIncome());
// All balance, that is not tied to rToken are dust (can be donated or is the rest of exponential vs linear)
return contractBalance <= totalRealBalance ? 0 : contractBalance - totalRealBalance;
}

First it retrieves the actual balance of the crvUSD in the RToken, then calculates the total real balance by retrieving the totalSupply:

function totalSupply() public view override(ERC20, IERC20) returns (uint256) {
return super.totalSupply().rayMul(ILendingPool(_reservePool).getNormalizedIncome());
}

However the total supply is already scaled by the liquidity index. But when calculating the real balance, it scales it again:
currentTotalSupply.rayMul(ILendingPool(_reservePool).getNormalizedIncome()); by the liquidity index. That means contractBalance will always be less than computed totalRealBalance, which will always return 0, because the real balance is scaled 2x by the index.

Impact

Dust amount can never be taken out from the RToken.

Tools Used

Manual Review

Recommendations

It should use rayDiv when calculating the real balance.

Updates

Lead Judging Commences

inallhonesty Lead Judge 7 months ago
Submission Judgement Published
Validated
Assigned finding tags:

RToken::calculateDustAmount incorrectly applies liquidity index, severely under-reporting dust amounts and permanently trapping crvUSD in contract

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